IEEE 802.11a is a protocol based on the 5.8GHz UNII band (instead of the 2.4GHz IsmBand), this protocol can reach theoretical speeds of 52Mbs.

In typical outdoor usage it will have a shorter range (though it may be effected by water less) however it's use of OFDM Modulation means it's better at dealing with MultiPathFading issues which are especially prevelant in warehouse / metropolitan area deployments due to reflections.

See also: ChannelFrequencyChart, 802.11b, 802.11g, AccessPointReviews

A recent posting by JimThompson to the PersonalTelco MailingList has more information:

{{{SamChurchill writes: > I thought 5 Ghz could deliver backbone services to multiple users 10 miles > away. Am I mistaken? > > A Multi-Point 5.8 Ghz tower on a hill reaching 10-15 miles might make a good > backbone for the Satellite Truck. But upon looking at the FCC part 15 rules > now I'm not so sure. The 5.8 Ghz band can go 20 or miles but only point to > point. Antenna gain limits EIRP for multipoint connections. Perhaps too much > to be useful. Am I wrong?}}}

( example of high sensitive card is the SR5 with -74dBm@54Mbps and -94dB@6Mbps on sale at Diswire